It is known that all types of traveling wave tubes (TWT's) tend to lose the desired synchronization between the electron beam and the interacting electromagnetic wave as the electron beam progresses along the slow wave structure (SWS). Such loss of beam-wave synchronization occurs at the expense of beam kinetic power and results in the loss of the desired traveling wave interaction, thereby limiting the attainable efficiencies.
Various methods have been proposed to delay the loss of synchronism and thereby enhance the efficiency of TWT's. One class of methods involves so-called velocity tapering, i.e., a gradual reduction in the SWS wave velocity near the output end of the TWT. With this approach the wave velocity and beam velocity decrease together, loss of synchronism is delayed, and TWT efficiency is thereby increased. In current practice the required velocity reduction is accomplished by a reduction in the periodic length of the SWS. This approach to TWT efficiency enhancement is discussed, for example, in the journal article "Improvement of Traveling Wave Tube Efficiency Through Period Tapering", N. H. Pond and R. J. Twiggs, IEEE Transactions on Electron Devices, Vol. ED-13, 1966, pp. 956-961.
Certain adverse effects limit the usefulness of period tapering as applied to cavity coupled traveling wave tubes. A large amount of period tapering may result in undesired oscillations in the CCTWT, as is discussed in more detail in the aforesaid Pond and Twiggs journal article. Furthermore, the reduction in periodic length provided by period tapering leads to an increase in Joule heating losses and a decrease in interaction impedance. Reference is made to "Calculation of Coupled-Cavity TWT Performance", J. R. M. Vaughn, IEEE Transactions on Electron Devices, Vol ED-22, 1975, pp. 880-890 for a further discussion of these effects.
A further approach to velocity tapering in TWT's having a coupled cavity slow wave structure is disclosed in U.S. Pat. No. 3,846,664 (King et al). In this patent the speed of travel of the applied r.f. wave is slowed so as to be substantially in step with the decreasing velocity of the electron beam by varying, in accordance with a predetermined tapering law, the resonant frequency of the coupling elements, i.e., slots, which couple the adjacent cavities of the slow wave structure. The main purpose of this approach is to provide frequency dependent velocity reduction so that higher frequencies have less velocity than lower frequencies with the result that the bandwidth is increased, i.e., the upper cut-off frequency remains the same and the lower cut-off frequency is reduced. A further patent of interest is U.S. Pat. No. 3,274,428 (Harris) which discloses a traveling wave tube having a band pass slow wave structure whose frequency characteristic varies along the length thereof, in order to inhibit oscillation. To accomplish this, the sizes of coupling apertures in partitions disposed transverse to the beam path are varied between maximum nearer the electron gun to a minimum nearer the collector electrode.